Prostheses of the aforementioned type are used in particular as knee joint or elbow joint prostheses. Owing to the high load placed on them by the body weight and owing to their complex movement pattern, namely the flexion as the main movement combined with a rotation of the tibia relative to the femur as a secondary movement, knee-joint protheses in particular are relatively susceptible to malfunction due to wear or disease. To treat this, knee joint prostheses are needed that preferably permit both movements, in the interest of restoring the joint function to as close as possible to the anatomical conditions. A further requirement of the joint prosthesis is that it must have sufficient stability, particularly if the support apparatus (ligaments, etc.) composed of soft-tissue parts is already weakened.
In the prior art, various knee prostheses have been disclosed with the aim of combining these partly contradictory objectives of, on the one hand, mobility about two axes and, on the other hand, sufficient stability. In a known rotary knee prosthesis (EP 0174531 B1), a tibial component and a femoral component are connected to each other by a coupling piece, which has an axial eye for receiving an axle carrying a femoral component, in order thereby to create a bending joint. The coupling piece also has a bearing bush which faces toward the tibia and into which a pin arranged on the tibial component engages, in order thereby to form a rotary joint. In order to create favourable friction conditions in this rotary bearing formed by pin and bearing bush, a sliding sleeve made of plastic is arranged in the bearing bush. It is cap-shaped, with a hemispherical top part covering the tip of the pin. The substantially cylindrical jacket bears on the wall of the coupling piece forming the bearing bush. This results in a chambering effect. However, it has been shown that jamming of the bearing sleeve can still occur. For the patient, this defect of the prosthesis generally entails revision surgery.
To overcome this disadvantage, an improved knee-joint endoprosthesis has been made known which is marketed under the name “Endo Modell” by Waldemar Link, Hamburg, Germany. In this, the metal sleeve is designed in two parts, wherein the plastics material providing good sliding properties is enclosed tightly by a shrunk-on metal sleeve with a plurality of drilled holes. The plastics material is better supported by this unit, such that the risk of a defect of the sliding sleeve, particularly on account of cold flow of the plastic as a result of high loading, is greatly reduced. In practice, however, it has been found that the now most important component in the direction of force flow, namely the coupling piece receiving the actual bearing bush, is susceptible to failure as a result of overloading. In a revision of the prosthesis, withdrawal of the bearing bush often proves difficult.